The invention relates to a process for increasing the molecular weight of polyamides and the polyamides obtainable by the process.
Polyamides are important thermoplastic materials from the group of industrial plastics of high strength, rigidity, hardness and heat deformation resistance. These mechanical and physical properties depend decisively on the molecular weight of the polymer. High-quality recycling of used polyamides and of production waste, for example from fibre production and the injection moulding sector, without an after-treatment is possible to only a limited extent because of reduced molecular weights.
Improvement in the material properties of used polyamides or polyamides which have been predamaged by heat or hydrolysis, the damage typically being accompanied by a reduction in molecular weight, is known in principle. Polyamides are accessible as polycondensation products, for example from post-condensation in the solid state (S. Fakirov, Kunststoffe 74 (1984), 218 and R. E. Gru/ tzner, A. Koine, Kunststoffe 82 (1992), 284). However, this method is protracted and furthermore reacts sensitively to impurities such as may be present in waste material.
The use of phosphoric acid or phosphorous or phosphonous acid as a catalyst has furthermore been proposed for solid phase condensation of polyamides in EP-0 410 230.
The object of the present invention was therefore development of a process which allows a substantial increase in the molecular weight of polyamides in a relatively short time.
Surprisingly, a substantial increase in the molecular weight of the polyamide is possible by melting the polyamide and by addition of a mixture of at least one bismaleimide and one sterically hindered hydroxyphenyl-alkyl-phosphonic acid ester or half-ester to the polyamide. This increase in molecular weight causes an improvement in the properties of the polyamides, in particular in the injection moulding sector and in recycled materials. An increase in molecular weight can be achieved with the aid of the process according to the invention on, in particular, polyamide recycled materials from collection of used industrial components of automobile and electrical applications.